1. Field of the Invention
The present invention relates to improvements in a fluid coupling, and more particularly to a fluid coupling which is used as a component of a cooling fan system for an internal combustion engine of a vehicle.
2. Description of the Prior Art
JP-A-60-241534 discloses a typical fluid coupling which is operated to adequately rotate a cooling fan for cooling an internal combustion engine. A casing and a cover are fixedly connected with each other and define a sealed space therebetween. A cooling fan is installed to the outer peripheral portion of the casing and cover of the fluid coupling. The sealed space is divided into a storage chamber and a torque transmission chamber by a partition plate having a hydraulic flow control hole. The drive disc having a disc portion and a shaft portion is sealingly connected to the casing through a bearing so that the disc portion is rotatably disposed in the torque transmission chamber. A valve member is attached on the partition plate so as to close and open the hydraulic flow control hole in accordance with the change of ambient temperature. The torque transmission from the drive disc to the cooling fan is controlled in a manner to change the connecting area between the drive disc and the wall of the casing through operating fluid. Furthermore, a weight member is attached to the free end portion of the valve member to decrease the opening degree of the hydraulic flow control hole in accordance with the increase of the rotating speed of the cooling fan. A fluid damming space is defined by a peripheral portion of the drive disc and a part of a wall defining the sealed space. A return passage is formed to communicate the torque transmission chamber and the storage chamber to circulate the operating fluid. Thus, this conventional arrangement solves problems that the rotating speed of the cooling fan is radically changed at a generally predetermined temperature condition. That is, this conventional arrangement prevents the occurrence of a fan noise and a degradation of fuel consumption due to the hunting phenomenon.
However, this arrangement is required in production to correctly adjust the rigidity of the valve member and the weight of the weight member since the control of the rotating speed of the cooling fan is carried out by utilizing a centrifugal force applied to the valve member. Additionally, this adjusting operation is difficult to be correctly carried out, and a production cost becomes high due to the increase of parts and the like.